Speaking at the Aachen conference on “Rapid Roadway Development,” Tom Lautsch of RAG Pennsylvania Service Corporation, analysed reasons for this lag and looked at ways of achieving better results.
The paper, entitled “Optimization of Multi-Entry Development Systems,” focused on the Appalachian region in the USA. One reason for this is because both methods of longwall development are in use in the region: in place entry development and place-changing.
In place-changing a continuous miner (CM) mines a cut to a distance of around 12m (the maximum allowed depth in the US) then moves to another cut, while a roof bolter supports the previously mined cut. In-place development is where the cutting and bolting operation is done with a single machine and no moves are required to advance the entry.
Lautsch said a comparison between both methods shows that place-change beats in-place methods in advance rate and costs. Place-change was not however suitable for mines with fairly incompetent roof conditions.
An analysis of the downtime affecting in-place development was undertaken at an Appalachian mine. Of the available shift time in total, downtime accounted for about 62% of the time. Of this total, organizational delays made up about a quarter, geofactors around 16%, bolter miner breakdowns 16% and section layout 10% of overall delays.
Around 39% of organizational delays are attributed to machine tramming and moving of cables while 25% is related to belt extension and moves of power centre. Of geotechnical downtimes, roof falls contribute 50% while 30% of delays in this category are caused by the bolter miner waiting for the roof bolting to catch up.
In the bolter miner breakdown category (16% of total downtime), problems with the hydraulic hoses are a major contributor of downtime.
Section layout contributes 10% to the total downtime, with three quarters of this time related to the cutting sequence of the bolter miner, as the machine cannot cut perpendicular crosscuts without significant productivity losses.
Mines seeking improvements in development were either interested in increasing the rate of advance, or in making these operations more cost effective.
Lautsch outlined four areas a longwall mine could target for improvements in roadway development.
Regarding the section layout (which is typically a three entry system in the Appalachian area), it was suggested that the optimum consists of a combination of angled crosscuts and special attention to ventilation to achieve an extended sequence length.
“A second point for major improvement is the change of the organizational structure within a longwall development section,” he said. “Simple arrangements like increasing the motivation of the crews, reducing the downtime of machines by means of better maintenance or improved supply chains and refitting processes are easy to implement.”
Introducing super sections were another potential organizational concept but required a big injection of capital, though the increased advance speed offset this investment.
Lautsch concluded: “The reduction of non productive shift time can be achieved easiest in the realization of less tramming time, minimized downtime due to belt moves and optimised cutting times, with the help of a different crosscut layout. The direct comparison of the two longwall development methods comes to the result that place-change offers advantages over the in-place method where the roof conditions allow for both alternatives.”